Catalytic reduction of NOx

ABSTRACT

A system for NO x  reduction in combustion gases, especially from diesel engines, incorporates an oxidation catalyst to convert at least a portion of NO to NO 2 , a particulate filter, a source of reductant such as NH 3  and an SCR catalyst. Considerable improvements in NO x  conversion are observed.

This application is a divisional application of U.S. patent application Ser. No. 09/601,964, filed Jan. 9, 2001, which is the U.S. national phase application of International Application No. PCT/GB99/00292, filed Jan. 28, 1999, and claims priority of British Patent Application No. 9802504.2, filed Feb. 6, 1998.

The present invention concerns improvements in selective catalytic reduction of NO_(x) in waste gas streams such as diesel engine exhausts or other lean exhaust gases such as from gasoline direct injection (GDI).

The technique named SCR (Selective Catalytic Reduction) is well established for industrial plant combustion gases, and may be broadly described as passing a hot exhaust gas over a catalyst in the presence of a nitrogenous reductant, especially ammonia or urea. This is effective to reduce the NO_(x) content of the exhaust gases by about 20-25% at about 250° C., or possibly rather higher using a platinum catalyst, although platinum catalysts tend to oxidise NH₃ to NO_(x) during higher temperature operation. We believe that SCR systems have been proposed for NO_(x) reduction for vehicle engine exhausts, especially large or heavy duty diesel engines, but this does require on-board storage of such reductants, and is not believed to have met with commercial acceptability at this time.

We believe that if there could be a significant improvement in performance of SCR systems, they would find wider usage and may be introduced into vehicular applications. It is an aim of the present invention to improve significantly the conversion of NO_(x) in a SCR system, and to improve the control of other pollutants using a SCR system.

Accordingly, the present invention provides an improved SCR catalyst system, comprising in combination and in order, an oxidation catalyst effective to convert NO to NO₂, a particulate filter, a source of reductant fluid and downstream of said source, an SCR catalyst.

The invention further provides an improved method of reducing NO_(x) in gas streams containing NO and particulates comprising passing such gas stream over an oxidation catalyst under conditions effective to convert at least a portion of NO in the gas stream to NO₂, removing at least a portion of said particulates, adding reductant fluid to the gas stream containing enhanced NO₂ to form a gas mixture, and passing the gas mixture over an SCR catalyst.

Although the present invention provides, at least in its preferred embodiments, the opportunity to reduce very significantly the NO_(x) emissions from the lean (high in oxygen) exhaust gases from diesel and similar engines, it is to be noted that the invention also permits very good reductions in the levels of other regulated pollutants, especially hydrocarbons and particulates.

The invention is believed to have particular application to the exhausts from heavy duty diesel engines, especially vehicle engines, e.g. truck or bus engines, but is not to be regarded as being limited thereto. Other applications might be LDD (light duty diesel), GDI, CNG (compressed natural gas) engines, ships or stationary sources. For simplicity, however, the majority of this description concerns such vehicle engines.

We have surprisingly found that a “pre-oxidising” step, which is not generally considered necessary because of the low content of CO and unburnt fuel in diesel exhausts, is particularly effective in increasing the conversion of NO_(x) to N₂ by the SCR system. We also believe that minimising the levels of hydrocarbons in the gases may assist in the conversion of NO to NO₂. This may be achieved catalytically and/or by engine design or management. Desirably, the NO₂/NO ratio is adjusted according to the present invention to the most beneficial such ratio for the particular SCR catalyst and CO and hydrocarbons are oxidized prior to the SCR catalyst. Thus, our preliminary results indicate that for a transition metal/zeolite SCR catalyst it is desirable to convert all NO to NO₂, whereas for a rare earth-based SCR catalyst, a high ratio is desirable providing there is some NO, and for other transition metal-based catalysts gas mixtures are notably better than either substantially only NO or NO₂. Even more surprisingly, the incorporation of a particulate filter permits still higher conversions of NO_(x).

The oxidation catalyst may be any suitable catalyst, and is generally available to those skilled in art. For example, a Pt catalyst deposited upon a ceramic or metal through-flow honeycomb support is particularly suitable. Suitable catalysts are e.g. Pt/A12O3 catalysts, containing 1-150 g Pt/ft³ (0.035-5.3 g Pt/litre) catalyst volume depending on the NO₂/NO ratio required. Such catalysts may contain other components providing there is a beneficial effect or at least no significant adverse effect.

The source of reductant fluid conveniently uses existing technology to inject fluid into the gas stream. For example, in the tests for the present invention, a mass controller was used to control supply of compressed NH₃, which was injected through an annular injector ring mounted in the exhaust pipe. The injector ring had a plurality of injection ports arranged around its periphery. A conventional diesel fuel injection system including pump and injector nozzle has been used to inject urea by the present applicants. A stream of compressed air was also injected around the nozzle; this provided good mixing and cooling.

The reductant fluid is suitably NH₃, but other reductant fluids including urea, ammonium carbamate and hydrocarbons including diesel fuel may also be considered. Diesel fuel is, of course, carried on board a diesel-powered vehicle, but diesel fuel itself is a less selective reductant than NH₃ and is presently not preferred.

Suitable SCR catalysts are available in the art and include Cu-based and vanadia-based catalysts. A preferred catalyst at present is a V₂O₅/WO₃/TiO₂ catalyst, supported on a honeycomb through-flow support. Although such a catalyst has shown good performance in the tests described hereafter and is commercially available, we have found that sustained high temperature operation can cause catalyst deactivation. Heavy duty diesel engines, which are almost exclusively turbocharged, can produce exhaust gases at greater than 500° C. under conditions of high load and/or high speed, and such temperatures are sufficient to cause catalyst deactivation. In one embodiment of the invention, therefore, cooling means is provided upstream of the SCR catalyst. Cooling means may suitably be activated by sensing high catalyst temperatures or by other, less direct, means, such as determining conditions likely to lead to high catalyst temperatures. Suitable cooling means include water injection upstream of the SCR catalyst, or air injection, for example utilising the engine turbocharger to provide a stream of fresh intake air by-passing the engine. We have observed a loss of activity of the catalyst, however, using water injection, and air injection by modifying the turbocharger leads to higher space velocity over the catalyst which tends to reduce NO_(x) conversion. Preferably, the preferred SCR catalyst is maintained at a temperature from 160° C. to 450° C.

We believe that in its presently preferred embodiments, the present invention may depend upon an incomplete conversion of NO to NO₂. Desirably, therefore, the oxidation catalyst, or the oxidation catalyst together with the particulate trap if used, yields a gas stream entering the SCR catalyst having a ratio of NO to NO₂ of from about 4:1 to about 1:3 by vol, for the commercial vanadia-type catalyst. As mentioned above, other SCR catalysts perform better with different NO/NO₂ ratios. We do not believe that it has previously been suggested to adjust the NO/NO₂ ratio in order to improve NO_(x) reduction.

The present invention incorporates a particulate trap downstream of the oxidation catalyst. We discovered that soot-type particulates may be removed from a particulate trap by “combustion” at relatively low temperatures in the presence of NO₂. In effect, the incorporation of such a particulate trap serves to clean the exhaust gas of particulates without causing accumulation, with resultant blockage or back-pressure problems, whilst simultaneously reducing a proportion of the NO_(x). Suitable particulate traps are generally available, and are desirably of the type known as wall-flow filters, generally manufactured from a ceramic, but other designs of particulate trap, including woven knitted or non-woven heat-resistant fabrics, may be used.

It may be desirable to incorporate a clean-up catalyst downstream of the SCR catalyst, to remove any NH₃ or derivatives thereof which could pass through unreacted or as by-products. Suitable clean-up catalysts are available to the skilled person.

A particularly interesting possibility arising from the present invention has especial application to light duty diesel engines (car and utility vehicles) and permits a significant reduction in volume and weight of the exhaust gas after-treatment system, in a suitable engineered system.

Several tests have been carried out in making the present invention. These are described below, and are supported by results shown in graphical form in the attached drawings.

A commercial 10 litre turbocharged heavy duty diesel engine on a test-bed was used for all the tests described herein.

Test 1—(Comparative)

A conventional SCR system using a commercial V₂O₅/WO₃/TiO₂ catalyst, was adapted and fitted to the exhaust system of the engine. NH₃ was injected upstream of the SCR catalyst at varying ratios. The NH₃ was supplied from a cylinder of compressed gas and a conventional mass flow controller used to control the flow of NH₃ gas to an experimental injection ring. The injection ring was a 10 cm diameter annular ring provided with 20 small injection ports arranged to inject gas in the direction of the exhaust gas flow. NO_(x) conversions were determined by fitting a NO_(x) analyser before and after the SCR catalyst and are plotted against exhaust gas temperature in FIG. 1. Temperatures were altered by maintaining the engine speed constant and altering the torque applied.

A number of tests were run at different quantities of NH₃ injection, from 60% to 100% of theoretical, calculated at 1:1 NH₃/NO and 4:3 NH₃/NO₂. It can readily be seen that at low temperatures, corresponding to light load, conversions are about 25%, and the highest conversions require stoichiometric (100%) addition of NH₃ at catalyst temperatures of from 325 to 400° C., and reach about 90%. However, we have determined that at greater than about 70% of stoichiometric NH₃ injection, NH₃ slips through the SCR catalyst unreacted, and can cause further pollution problems.

Test 2 (Comparative)

The test rig was modified by inserting into the exhaust pipe upstream of the NH₃ injection, a commercial platinum oxidation catalyst of 10.5 inch diameter and 6 inch length (26.67 cm diameter and 15.24 cm length) containing 10 g Pt/ft³ (=0.35 g/litre) of catalyst volume. Identical tests were run, and it was observed from the results plotted in FIG. 2, that even at 225° C., the conversion of NO_(x) has increased from 25% to >60%. The greatest conversions were in excess of 95%. No slippage of NH₃ was observed in this test nor in the following test.

Test 3

The test rig was modified further, by inserting a particulate trap before the NH₃ injection point, and the tests run again under the same conditions at 100% NH₃ injection and a space velocity in the range 40,000 to 70,000 hr⁻¹ over the SCR catalyst. The results are plotted and shown in FIG. 3. Surprisingly, there is a dramatic improvement in NO_(x) conversion, to above 90% at 225° C., and reaching 100% at 350° C. Additionally, of course, the particulates which are the most visible pollutant from diesel engines, are also controlled.

Test 4

An R49 test with 80% NH₃ injection was carried out over a V₂O₅/WO3/TiO₂ SCR catalyst. This gave 67% particulate, 89% HC and 87% NO_(x) conversion; the results are plotted in FIG. 4.

Additionally tests have been carried out with a different diesel engine, and the excellent results illustrated in Test 3 and 4 above have been confirmed.

The results have been confirmed also for a non-vanadium SCR catalyst. 

1.-14. (canceled)
 15. A method of reducing NO_(x) in a gas stream containing NO and particulates, comprising: passing the gas stream over an oxidation catalyst thereby converting at least a portion of the NO in the gas stream to NO₂ to form a converted gas stream; adding a reductant fluid to the converted gas stream to form a gas mixture; and passing the gas mixture over an SCR catalyst.
 16. The method of claim 15, wherein the ratio of NO to NO₂ in the gas mixture is from about 4:1 to 1:3 by volume.
 17. The method of claim 15, wherein the gas stream comprises exhaust from sources selected from the group consisting of: heavy duty diesel engines, light duty diesel engines, gasoline direct injection engines, compressed natural gas engines, ships, and stationary sources.
 18. The method of claim 17, wherein the gas stream comprises exhaust from a heavy duty diesel engine.
 19. The method of claim 17, wherein the gas stream comprises exhaust from a light duty diesel engine.
 20. The method of claim 17, wherein the gas stream comprises exhaust from a gasoline direct injection engine.
 21. The method of claim 17, wherein the gas stream comprises exhaust from a compressed natural gas engine.
 22. The method of claim 15, wherein the SCR catalyst is selected from the group consisting of transition metal/zeolite catalysts, rare earth-based catalysts and transition metal catalysts.
 23. The method of claim 15, wherein the SCR catalyst comprises a transition metal/zeolite catalyst.
 24. The method of claim 15, wherein the oxidation catalyst comprises a platinum catalyst.
 25. The method of claim 24, wherein the platinum catalyst is deposited on a ceramic through-flow honeycomb support.
 26. The method of claim 24, wherein the oxidation catalyst comprises platinum deposited on a metal through-flow honeycomb support.
 27. The method of claim 24, wherein the platinum catalyst comprises a Pt/Al₂O₃ catalyst.
 28. The method of claim 15, wherein the reductant fluid is selected from the group consisting of NH₃, urea, ammonium carbamate, hydrocarbons, and diesel fuel.
 29. The method of claim 15, wherein the reductant fluid comprises NH₃.
 30. The method of claim 15, wherein the reductant fluid comprises urea.
 31. The method of claim 15, wherein the reductant fluid comprises ammonium carbamate.
 32. The method of claim 15, wherein the reductant fluid is injected into the gas stream.
 33. The method of claim 32, wherein the supply of the reductant fluid is controlled using a mass controller.
 34. The method of claim 32, wherein the reductant fluid is injected into the gas stream through an injection ring.
 35. The method of claim 34, wherein the injection ring is an annular injection ring mounted in an exhaust pipe of a vehicle.
 36. The method of claim 15, wherein the SCR catalyst is maintained at a temperature of from 160° C. to 450° C.
 37. The method of claim 36, wherein the temperature of the catalyst is maintained using a cooling means.
 38. The method of claim 37, wherein the cooling means comprises water injection.
 39. The method of claim 37, wherein the cooling means comprises air injection.
 40. The method of claim 15, wherein the ratio of the reductant fluid to NO added to the converted gas stream comprises 0.6:1 to 1:1.
 41. The method of claim 40, wherein the reductant fluid comprises NH₃.
 42. The method of claim 15, wherein the ratio of the reductant fluid to NO₂ added to the converted gas stream comprises 0.8:1 to 4:3.
 43. The method of claim 42, wherein the reductant fluid comprises NH₃.
 44. The method of claim 41, further comprising removing any NH₃ and derivatives thereof downstream of the SCR catalyst.
 45. The method of claim 44, wherein the removing any NH₃ and derivatives thereof downstream of the SCR catalyst includes incorporation of a clean-up catalyst downstream of the SCR catalyst.
 46. A method according to claim 15, wherein a space velocity of the exhaust gas over the SCR catalyst is in the range 40,000 to 70,000 hr⁻¹.
 47. A method of reducing NO_(x) in a gas stream containing NO and particulates, comprising: passing the gas stream over an oxidation catalyst thereby converting at least a portion of the NO in the gas stream to NO₂ to form a converted gas stream; removing at least a portion of the particulates from the converted gas stream; adding a reductant fluid to the converted gas stream to form a gas mixture; and passing the gas mixture over an SCR catalyst.
 48. The method of claim 47, wherein the particulates are removed from the converted gas stream using a particulate filter or particulate trap.
 49. The method of claim 48, wherein the particulates are removed without causing accumulation and resulting blockage and back pressure problems.
 50. The method of claim 48, wherein the particles are removed from the particulate trap by combustion in the presence of NO₂.
 51. The method of claim 48, wherein the particulate trap comprises a wall-flow filter.
 52. The method of claim 48, wherein the particulate trap is manufactured from ceramic.
 53. The method of claim 48, wherein the particulate trap is manufactured from woven knitted heat resistant fabrics.
 54. The method of claim 48, wherein the particulate trap is manufactured from non-woven heat resistant fabrics.
 55. The method of claim 48, wherein the ratio of NO to NO₂ in the gas mixture is from about 4:1 to 1:3 by volume.
 56. The method of claim 48, wherein the gas stream comprises exhaust from sources selected from the group consisting of: heavy duty diesel engines, light duty diesel engines, gasoline direct injection engines, compressed natural gas engines, ships, and stationary sources.
 57. The method of claim 56, wherein the gas stream comprises exhaust from a heavy duty diesel engine.
 58. The method of claim 56, wherein the gas stream comprises exhaust from a light duty diesel engine.
 59. The method of claim 56, wherein the gas stream comprises exhaust from a gasoline direct injection engine.
 60. The method of claim 48, wherein the SCR catalyst is selected from the group consisting of transition metal/zeolite catalysts, rare earth-based catalysts and transition metal catalysts.
 61. The method of claim 48, wherein the SCR catalyst comprises a transition metal/zeolite catalyst.
 62. The method of claim 48, wherein the oxidation catalyst comprises a platinum catalyst.
 63. The method of claim 62, wherein the platinum catalyst is deposited on a ceramic through-flow honeycomb support.
 64. The method of claim 62, wherein the platinum catalyst is deposited on a metal through-flow honeycomb support.
 65. The method of claim 62, wherein the platinum catalyst comprises a Pt/Al₂O₃ catalyst.
 66. The method of claim 48, wherein the reductant fluid is selected from the group consisting of NH₃, urea, ammonium carbamate, hydrocarbons, and diesel fuel.
 67. The method of claim 66, wherein the reductant fluid comprises NH₃.
 68. The method of claim 66, wherein the reductant fluid comprises urea.
 69. The method of claim 66, wherein the reductant fluid comprises ammonium carbamate.
 70. The method of claim 48, wherein the SCR catalyst is maintained at a temperature of from 160° C. to 450° C.
 71. The method of claim 70, wherein the temperature of the catalyst is maintained using a cooling means.
 72. The method of claim 71, wherein the cooling means comprises water injection.
 73. The method of claim 71, wherein the cooling means comprises air injection.
 74. The method of claim 48, wherein the ratio of the reductant fluid to NO added to the gas stream comprises 0.6:1 to 1:1.
 75. The method of claim 54, wherein the reductant fluid comprises NH₃.
 76. The method of claim 48, wherein the ratio of the reductant fluid to NO₂ added to the gas stream comprises 0.8:1 to 4:3.
 77. The method of claim 76, wherein the reductant fluid comprises NH₃.
 78. The method of claim 75, further comprising removing any NH₃ and derivatives thereof downstream of the SCR catalyst.
 79. The method of claim 64, wherein the removing any NH₃ and derivatives thereof downstream of the SCR catalyst includes incorporation of a clean-up catalyst downstream of the SCR catalyst.
 80. A method according to claim 48, wherein the space velocity of the exhaust gas over the SCR catalyst is in the range 40,000 to 70,000 hr⁻¹.
 81. A method of reducing NO_(x) in a gas stream containing NO and particulates, comprising: passing the gas stream over an oxidation catalyst thereby converting at least a portion of the NO in the gas stream to NO₂ to form a converted gas stream having a ratio of NO to NO₂ adjusted according to the type of SCR catalyst to improve NO_(x) reduction; adding a reductant fluid to the converted gas stream to form a gas mixture; and passing the gas mixture over an SCR catalyst.
 82. The method of claim 81, wherein the ratio of NO to NO₂ in the gas mixture is from about 4:1 to 1:3 by volume.
 83. The method of claim 81, wherein the gas stream comprises exhaust from sources selected from the group consisting of: heavy duty diesel engines, light duty diesel engines, gasoline direct injection engines, compressed natural gas engines, ships, and stationary sources.
 84. The method of claim 81, wherein the SCR catalyst comprises a transition metal/zeolite catalyst.
 85. The method of claim 81, wherein the oxidation catalyst comprises a platinum catalyst deposited on a ceramic through-flow honeycomb support.
 86. The method of claim 85, wherein the platinum catalyst comprises a Pt/Al₂O₃ catalyst.
 87. The method of claim 81, wherein the reductant fluid comprises urea.
 88. A method of reducing NO_(x) in a gas stream containing NO and particulates, comprising: passing the gas stream over an oxidation catalyst thereby converting at least a portion of the NO in the gas stream to NO₂ to form a converted gas stream having a ratio of NO to NO₂ adjusted according to the type of SCR catalyst to improve NO_(x) reduction; removing at least a portion of the particulates from the converted gas stream; adding a reductant fluid to the converted gas stream to form a gas mixture; and passing the gas mixture over an SCR catalyst.
 89. The method of claim 88, wherein the ratio of NO to NO₂ in the gas mixture is from about 4:1 to 1:3 by volume.
 90. The method of claim 88, wherein the gas stream comprises exhaust from sources selected from the group consisting of: heavy duty diesel engines, light duty diesel engines, gasoline direct injection engines, compressed natural gas engines, ships, and stationary sources.
 91. The method of claim 88, wherein the SCR catalyst comprises a transition metal/zeolite catalyst.
 92. The method of claim 88, wherein the oxidation catalyst comprises a platinum catalyst deposited on a ceramic through-flow honeycomb support.
 93. The method of claim 92, wherein the platinum catalyst comprises a Pt/Al₂O₃ catalyst.
 94. The method of claim 88, wherein the reductant fluid comprises urea.
 95. A method of improving NO_(x) conversion in an SCR system, comprising: passing the gas stream over an oxidation catalyst thereby converting at least a portion of the NO in the gas stream to NO₂ to form a converted gas stream; adding a reductant fluid to the converted gas stream to form a gas mixture; and passing the gas mixture over an SCR catalyst.
 96. The method of claim 95, wherein the ratio of NO to NO₂ in the gas mixture is from about 4:1 to 1:3 by volume.
 97. The method of claim 95, wherein the gas stream comprises exhaust from sources selected from the group consisting of: heavy duty diesel engines, light duty diesel engines, gasoline direct injection engines, compressed natural gas engines, ships, and stationary sources.
 98. The method of claim 95, wherein the SCR catalyst comprises a transition metal/zeolite catalyst.
 99. The method of claim 95, wherein the oxidation catalyst comprises a platinum catalyst deposited on a ceramic through-flow honeycomb support.
 100. The method of claim 99, wherein the platinum catalyst comprises a Pt/Al₂O₃ catalyst.
 101. The method of claim 95, wherein the reductant fluid comprises urea.
 102. A method of improving NO_(x) conversion in an SCR system, comprising: passing the gas stream over an oxidation catalyst thereby converting at least a portion of the NO in the gas stream to NO₂ to form a converted gas stream; removing at least a portion of the particulates from the converted gas stream; adding a reductant fluid to the converted gas stream to form a gas mixture; and passing the gas mixture over an SCR catalyst.
 103. The method of claim 102, wherein the ratio of NO to NO₂ in the gas mixture is from about 4:1 to 1:3 by volume.
 104. The method of claim 102, wherein the gas stream comprises exhaust from sources selected from the group consisting of: heavy duty diesel engines, light duty diesel engines, gasoline direct injection engines, compressed natural gas engines, ships, and stationary sources.
 105. The method of claim 102, wherein the SCR catalyst comprises a transition metal/zeolite catalyst.
 106. The method of claim 102, wherein the oxidation catalyst comprises a platinum catalyst deposited on a ceramic through-flow honeycomb support.
 107. The method of claim 106, wherein the platinum catalyst comprises a Pt/Al₂O₃ catalyst.
 108. The method of claim 102, wherein the reductant fluid comprises urea.
 109. A process for reducing NO_(x) present in a lean exhaust gas from an internal combustion engine by selective catalytic reduction on a reduction catalyst using ammonia, comprising oxidizing some of the NO present in the exhaust gas to NO₂ so that the ratio of NO to NO₂ in the exhaust gas is from about 4:1 to 1:3 by volume before contact with the reduction catalyst, wherein oxidation of the NO present in the exhaust gas takes place in the presence of an oxidation catalyst, passing the exhaust gas, together with ammonia, over said reduction catalyst, wherein the reduction catalyst comprises a transition metal/zeolite catalyst.
 110. The process according to claim 109, wherein the oxidation catalyst comprises platinum on aluminum oxide.
 111. The process according to claim 110, wherein the oxidation catalyst is deposited on a honeycomb carrier.
 112. A method of reducing levels of regulated pollutants in a gas stream comprising: passing the gas stream over an oxidation catalyst thereby converting at least a portion of the NO in the gas stream to NO₂ to form a converted gas stream; adding a reductant fluid to the converted gas stream to form a gas mixture; and passing the gas mixture over an SCR catalyst.
 113. The method of claim 112, wherein the ratio of NO to NO₂ in the gas mixture is from about 4:1 to 1:3 by volume.
 114. The method of claim 112, wherein the gas stream comprises exhaust from sources selected from the group consisting of: heavy duty diesel engines, light duty diesel engines, gasoline direct injection engines, compressed natural gas engines, ships, and stationary sources.
 115. The method of claim 112, wherein the SCR catalyst comprises a transition metal/zeolite catalyst.
 116. The method of claim 112, wherein the oxidation catalyst comprises a platinum catalyst deposited on a ceramic through-flow honeycomb support.
 117. The method of claim 116, wherein the platinum catalyst comprises a Pt/Al₂O₃ catalyst.
 118. The method of claim 112, wherein the reductant fluid comprises urea.
 119. The method according to claim 112, wherein the regulated pollutants comprise particulates.
 120. The method according to claim 112, wherein the regulated pollutants comprise hydrocarbons.
 121. A method of reducing levels of regulated pollutants in a gas stream comprising: passing the gas stream over an oxidation catalyst thereby converting at least a portion of the NO in the gas stream to NO₂ to form a converted gas stream; removing at least a portion of the particulates from the converted gas stream; adding a reductant fluid to the converted gas stream to form a gas mixture; and passing the gas mixture over an SCR catalyst.
 122. The method according to claim 121, wherein the regulated pollutants comprise particulates.
 123. The method according to claim 121, wherein the regulated pollutants comprise hydrocarbons.
 124. The method of claim 121, wherein the ratio of NO to NO₂ in the gas mixture is from about 4:1 to 1:3 by volume.
 125. The method of claim 121, wherein the gas stream comprises exhaust from sources selected from the group consisting of: heavy duty diesel engines, light duty diesel engines, gasoline direct injection engines, compressed natural gas engines, ships, and stationary sources.
 126. The method of claim 121, wherein the SCR catalyst comprises a transition metal/zeolite catalyst.
 127. The method of claim 121, wherein the oxidation catalyst comprises a platinum catalyst deposited on a ceramic through-flow honeycomb support.
 128. The method of claim 127, wherein the platinum catalyst comprises a Pt/Al₂O₃ catalyst.
 129. The method of claim 121, wherein the reductant fluid comprises urea.
 130. A method of improving NO_(x) conversion in an SCR system, comprising: passing the gas stream over an oxidation catalyst thereby converting at least a portion of the NO in the gas stream to NO₂ to form a converted gas stream; removing at least a portion of the particulates from the converted gas stream; minimizing the level of hydrocarbons in the gas stream; adding a reductant fluid to the converted gas stream to form a gas mixture; and passing the gas mixture over an SCR catalyst.
 131. The method of claim 130, wherein the ratio of NO to NO₂ in the gas mixture is from about 4:1 to 1:3 by volume.
 132. The method of claim 130, wherein the gas stream comprises exhaust from sources selected from the group consisting of: heavy duty diesel engines, light duty diesel engines, gasoline direct injection engines, compressed natural gas engines, ships, and stationary sources.
 133. The method of claim 130, wherein the SCR catalyst comprises a transition metal/zeolite catalyst.
 134. The method of claim 130, wherein the oxidation catalyst comprises a platinum catalyst deposited on a ceramic through-flow honeycomb support.
 135. The method of claim 134, wherein the platinum catalyst comprises a Pt/Al₂O₃ catalyst.
 136. The method of claim 130, wherein the reductant fluid comprises urea.
 137. A method of reducing NO_(x) in a gas stream comprising: passing the gas stream over an oxidation catalyst thereby incompletely converting NO in the gas stream to NO₂ to form a converted gas stream; adding a reductant fluid to the converted gas stream to form a gas mixture; and passing the gas mixture over an SCR catalyst.
 138. The method of claim 137, wherein the ratio of NO to NO₂ in the gas mixture is from about 4:1 to 1:3 by volume.
 139. The method of claim 137, wherein the gas stream comprises exhaust from sources selected from the group consisting of: heavy duty diesel engines, light duty diesel engines, gasoline direct injection engines, compressed natural gas engines, ships, and stationary sources.
 140. The method of claim 137, wherein the SCR catalyst comprises a transition metal/zeolite catalyst.
 141. The method of claim 137, wherein the oxidation catalyst comprises a platinum catalyst deposited on a ceramic through-flow honeycomb support.
 142. The method of claim 141, wherein the platinum catalyst comprises a Pt/Al₂O₃ catalyst.
 143. The method of claim 137, wherein the reductant fluid comprises urea.
 144. A method of reducing volume and/or weight of an exhaust gas after-treatment system of a light duty diesel engine comprising: attaching an SCR system to the light duty diesel engine, the SCR system providing reduction of NO_(x) in the exhaust gas by: passing the exhaust gas over an oxidation catalyst thereby converting at least a portion of the NO in the exhaust gas to NO₂ to form a converted exhaust gas; adding a reductant fluid to the converted gas stream to form a gas mixture; and passing the gas mixture over an SCR catalyst.
 145. The method of claim 144, wherein the ratio of NO to NO₂ in the gas mixture is from about 4:1 to 1:3 by volume.
 146. The method of claim 144, wherein the gas stream comprises exhaust from sources selected from the group consisting of: heavy duty diesel engines, light duty diesel engines, gasoline direct injection engines, compressed natural gas engines, ships, and stationary sources.
 147. The method of claim 144, wherein the SCR catalyst comprises a transition metal/zeolite catalyst.
 148. The method of claim 144, wherein the oxidation catalyst comprises a platinum catalyst deposited on a ceramic through-flow honeycomb support.
 149. The method of claim 148, wherein the platinum catalyst comprises a Pt/Al₂O₃ catalyst.
 150. The method of claim 144, wherein the reductant fluid comprises urea.
 151. A method of reducing NO_(x) in a gas stream containing NO and particulates, comprising: passing the gas stream over an oxidation catalyst thereby converting at least a portion of the NO in the gas stream to NO₂ to form a converted gas stream; removing at least a portion of the particulates from the converted gas stream; and passing the gas mixture over an SCR catalyst.
 152. The method of claim 151, wherein the particulates are removed from the converted gas stream using a particulate filter or particulate trap.
 153. The method of claim 151, wherein the ratio of NO to NO₂ in the gas mixture is from about 4:1 to 1:3 by volume.
 154. The method of claim 151, wherein the gas stream comprises exhaust from sources selected from the group consisting of: heavy duty diesel engines, light duty diesel engines, gasoline direct injection engines, compressed natural gas engines, ships, and stationary sources.
 155. The method of claim 151, wherein the SCR catalyst comprises a transition metal/zeolite catalyst.
 156. The method of claim 151, wherein the oxidation catalyst comprises a platinum catalyst deposited on a ceramic through-flow honeycomb support.
 157. The method of claim 156, wherein the platinum catalyst comprises a Pt/Al₂O₃ catalyst.
 158. The method of claim 151, wherein the reductant fluid comprises urea.
 159. A method of reducing pollutants, including particulates and NO_(x), in a gas stream, comprising passing said gas stream over an oxidation catalyst under conditions effective to convert at least a portion of NO in the gas stream to NO₂ thereby enhancing the NO₂ content of the gas stream, removing at least a portion of said particulates in a particulate trap, reacting trapped particulate with NO₂, adding reductant fluid to the gas stream to form a gas mixture downstream of said trap, and passing the gas mixture over an SCR catalyst under NO_(x) reduction conditions.
 160. A method according to claim 159, wherein said gas stream is the exhaust from a diesel, GDI or DNG engine.
 161. A method according to claim 159, wherein the gas stream or gas mixture is cooled before reaching the SCR catalyst.
 162. A method according to claim 159, wherein the NO to NO₂ ratio of the gas mixture is adjusted to a level pre-determined to be optimum for the SCR catalyst, by oxidation of NO over said oxidation catalyst.
 163. A method according to claim 159, wherein the SCR catalyst is maintained at a temperature from 160° C. to 450° C.
 164. A method according to claim 159, wherein the SCR catalyst includes a component selected from the group consisting of a transition metal and a rare-earth metal.
 165. A method according to claim 164, wherein the transition metal is selected from the group consisting of copper and vanadium.
 166. A method according to claim 159, wherein the SCR catalyst is V₂O₅/WO₃/TiO₂.
 167. A method according to claim 162, wherein substantially all NO is converted to NO₂.
 168. A method according to claim 162, wherein the ratio of NO:NO₂ is adjusted to about 4:3.
 169. A method according to claim 159, wherein the reductant fluid is a hydrocarbon.
 170. A method according to claim 159, wherein the reductant fluid is selected from the group consisting of ammonia, ammonium carbamate and urea.
 171. A method according to claim 168, comprising contacting the gas mixture leaving the SCR catalyst with a clean-up catalyst to remove NH₃ or derivatives thereof.
 172. A method according to claim 171, wherein the space velocity of the exhaust gas over the SCR catalyst is in the range 40,000 to 70,000 hr⁻¹.
 173. An SCR system for reducing NO_(x) in exhaust gases comprising: an oxidation catalyst that converts at least a portion of NO in the exhaust gas to NO₂; a reductant fluid source downstream from the oxidation catalyst; and an SCR catalyst downstream from the reductant fluid source.
 174. The system of claim 173, wherein the ratio of NO to NO₂ in the exhaust gas just prior to the SCR catalyst is from about 4:1 to 1:3 by volume.
 175. The system of claim 173, wherein the exhaust gas comprises exhaust from sources selected from the group consisting of: heavy duty diesel engines, light duty diesel engines, gasoline direct injection engines, compressed natural gas engines, ships, and stationary sources.
 176. The system of claim 173, wherein the SCR catalyst comprises a transition metal/zeolite catalyst.
 177. The system of claim 173, wherein the oxidation catalyst comprises a platinum catalyst deposited on a ceramic through-flow honeycomb support.
 178. The system of claim 177, wherein the platinum catalyst comprises a Pt/Al₂O₃ catalyst.
 179. The system of claim 173, wherein the reductant fluid comprises urea.
 180. The system of claim 173, wherein the ratio of the reductant fluid to NO in the exhaust gas stream just prior to the SCR catalyst comprises 0.6:1 to 1:1.
 181. The system of claim 173, wherein the ratio of the reductant fluid to NO₂ in the exhaust gas stream just prior to the SCR catalyst comprises 0.8:1 to 4:3.
 182. The system of claim 173 further comprising a clean-up catalyst downstream of the SCR catalyst.
 183. The system of claim 173, wherein a space velocity capacity of the SCR catalyst is in the range of 40,000 to 70,000 hr⁻¹.
 184. An SCR system for reducing NO_(x) in exhaust gases comprising: an oxidation catalyst that converts at least a portion of NO in the exhaust gas to NO₂; a particulate filter or particulate trap downstream from the oxidation catalyst; a reductant fluid source downstream from the particulate filter or particulate trap; and an SCR catalyst downstream from the reductant fluid source.
 185. The system of claim 184, wherein the ratio of NO to NO₂ in the exhaust gas just prior to the SCR catalyst is from about 4:1 to 1:3 by volume.
 186. The system of claim 184, wherein the exhaust gas comprises exhaust from sources selected from the group consisting of: heavy duty diesel engines, light duty diesel engines, gasoline direct injection engines, compressed natural gas engines, ships, and stationary sources.
 187. The system of claim 184, wherein the SCR catalyst comprises a transition metal/zeolite catalyst.
 188. The system of claim 184, wherein the oxidation catalyst comprises a platinum catalyst deposited on a ceramic through-flow honeycomb support.
 189. The system of claim 184, wherein the platinum catalyst comprises a Pt/Al₂O₃ catalyst.
 190. The system of claim 184, wherein the reductant fluid comprises urea.
 191. The system of claim 184, wherein the ratio of the reductant fluid to NO in the exhaust gas stream just prior to the SCR catalyst comprises 0.6:1 to 1:1.
 192. The system of claim 184, wherein the ratio of the reductant fluid to NO₂ in the exhaust gas stream just prior to the SCR catalyst comprises 0.8:1 to 4:3.
 193. The system of claim 184 further comprising a clean-up catalyst downstream of the SCR catalyst.
 194. The system of claim 184, wherein a space velocity capacity of the SCR catalyst is in the range of 40,000 to 70,000 hr⁻¹. 